Fisher 234 FM Stereo Lock Issue

Discussion in 'Fisher' started by Rybo Flavin, Nov 6, 2018.

  1. Rybo Flavin

    Rybo Flavin New Member

    Last spring I picked up an excellent condition Fisher 234 receiver. I realize it's not a valuable unit, but it was the same model my family had while growing up and I have a lot of fond memories with it, so I would love to get it working properly. Also, this is a good opportunity for me to dust off some electronics knowledge that I have haven't used since college 20 years ago.

    I've spent a lot of time over the last month studying the service manual, reading forum posts, and watching Youtube videos in order to get an understanding of how the circuitry works. So, please have patience with me if I don't use the correct terminology or have a misunderstanding in the below problem description.

    For reference, here is the service manual with complete schematics and tuner description.

    The Problem

    Everything on the unit works correctly except it will not lock onto a FM stereo signal properly and I was hoping to get some suggestions. Here is a short video of the behavior (this is one of the strongest stations in my area, not that I listen to it regularly):

    As you can see, the FM stereo will attempt to lock in accordance with changes in the audio amplitude (especially lower frequency audio). This behavior will happen for a few minutes, then it will go into mono operation for a few minutes, then it will return to the prior behavior. By the way, the tuner is picking up stations better than every other radio in the house and the sound quality in mono mode is quite good and both speakers have the same volume level. Pressing the mono button will forcefully place the audio into mono mode as expected.

    In looking at the tuner board, the first thing I noticed was that, at some point, someone changed the MPX IC502 chip from a TR09027 to a SN76115N. Thinking I could get off easy, I replaced this chip with a new SN76115N but doing so did not make any difference.

    At this point I started taking voltage readings (see voltages below). I currently don't have an oscilloscope, but I have digital and analog multimeters and a nice soldering station. I made one interesting observation when using the analog meter on the MPX pin caused the FM stereo to solidly lock. I'm assuming this is due to extra resistance placed into the circuit by the meter. When it went into locked FM stereo mode, the right speaker got much louder and clearer, but the left speaker was only half as loud as the right. This may be a different, unrelated problem to be addressed after correcting the stereo lock issue.

    This leads me to believe the problem is occurring somewhere in the muting circuit, but this is where I need some help because I don't quite understand how this circuit should work in correlation to the actual voltage readings and I don't have the experience to know what to test/replace next. My understanding is that IC501 pin 12 feeds a positive "control voltage" to IC502 pin 9 to disable the stereo when the signal is too noisy. Perhaps the IC501 pin 12 voltage of 671mV is too high?...if so I don't know how to get it lower. IC501 is a TR09018 which I was unable to find a datasheet, but it seems the equivalent is the NTE788.

    I'm happy to provide any further info needed to troubleshoot. Please see the voltage readings below:

    Voltages When FM Stereo is Fluttering
    1 = 2.13V 9 = 5.61V
    2 = 2.12V 10 = 5.61V
    3 = 2.13V 11 = 11.71V
    4 = 0 12 = 671mV
    5 = 15.7mV 13 = 2.64V
    6 = 6.32V 14 = 0
    7 = 6.04V 15 = 3.95V
    8 = 5.61V 16 = 0

    1 = 11.12V 8 = 2.25V
    2 = 2.94V 9 = 2.236V
    3 = 4.51V 10 = 108mV
    4 = 6.48V 11 = 2.25V
    5 = 6.48V 12 = 2.238V
    6 = .20mV 13 = 2.256V
    7 = 0 14 = 3.72V

    B = .392mV
    C = 7.31V

    B = 628mV
    C = 51.7mV

    51mV == 2.192V

    14.18V == 51mV

    7.32V == 14.16V

    2.235V == 2.234V
    Last edited: Nov 6, 2018


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  2. r_brumett61

    r_brumett61 Active Member

    An "o" scope would be a big help. Did this just happen or have you changed/checked your FM antenna connections. It might also need a complete FM adj because of age drift and new IC's.
  3. Rybo Flavin

    Rybo Flavin New Member

    I've been wanting an o-scope for a while, guess now may be the time. Anyone have a recommendation for a decent unit for occasional use?

    It's been doing this since I bought the unit last spring. I've tried different antennas and even taking it out into my driveway to get the best reception, but this behavior remained consistent.

    The FM IF IC501 chip appears to be the original TR09018, so only the MPX decoder IC502 has been changed. From what I've read, it sounds like FM adjustment is a tricky process even if you have the right equipment (which I do not). I was going under the assumption that since this unit is picking up FM stations so strongly, that the FM adjustment was probably good.

    I was hoping that a simple bad capacitor / transistor / diode / resistor could be causing this. Perhaps it's heat related since it goes from mono-only mode to stereo-fluttering mode periodically?

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